Conical-cavity fiber optic sensor for temperature measurement in a steel furnace

An integrating-cavity sensor for temperature measurement of the steel sheet in a continuous annealing furnace is described. The sensor includes a fiber optic cable for transmission of the infrared radiation from the sensor head within the furnace to the double-wavelength detection unit located outside the furnace. The sensor head includes a reflecting conical cavity positioned above the steel sheet to increase, and thus stabilize, the effective infrared emissivity of the sheet surface as well as to reduce spurious reflections from other sources within the furnace. The fiber optic cable is both decentered and tilted with respect to the cavity axis to minimize straight-reflection losses through the fiber cable gate. Preliminary results after in-furnace installation are described.